Use channel's real funding amounts when processing RGS data

[TheBlueMatt]

Neither channel_announcements nor channel_updates contain a channel's actual funding amount, complicating scoring as nodes may set an htlc_maximum_msat to something less than the funding amount. When scoring, we use htlc_maximum_msat anyway if we don't know the funding amount, but its not a perfect proxy.

In
lightningdevkit/rapid-gossip-sync-server#102 we started including a channel's real funding amount in RGS data, and here we start parsing it and including it in our network graph.

Fixes #1559

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.56%. Comparing base (d67bd0f) to head (7ac02a6).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-rapid-gossip-sync/src/processing.rs	88.23%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3924      +/-   ##
==========================================
+ Coverage   88.83%   89.56%   +0.73%     
==========================================
  Files         166      166              
  Lines      119259   128662    +9403     
  Branches   119259   128662    +9403     
==========================================
+ Hits       105940   115241    +9301     
- Misses      10992    11032      +40     
- Partials     2327     2389      +62     

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[TheBlueMatt]

Fixed the spurious error acceptance:

$ git diff-tree -U2 7ac02a625 7ad6364ab
diff --git a/lightning-rapid-gossip-sync/src/processing.rs b/lightning-rapid-gossip-sync/src/processing.rs
index 7670147ef7..002807414b 100644
--- a/lightning-rapid-gossip-sync/src/processing.rs
+++ b/lightning-rapid-gossip-sync/src/processing.rs
@@ -275,5 +275,6 @@ where
 				let mut cursor = &additional_data[..];
 				let funding_sats_read: Result<BigSize, _> = Readable::read(&mut cursor);
-				funding_sats = funding_sats_read.map(|sats| Some(sats.0)).unwrap_or(None);
+				funding_sats =
+					funding_sats_read.map(|sats| Some(sats.0)).ok_or(DecodeError::InvalidValue)?;
 				if !cursor.is_empty() {
 					log_gossip!(

[tnull]

Seems CI is unhappy:

error[E0599]: no method named `ok_or` found for enum `Result` in the current scope
   --> lightning-rapid-gossip-sync/src/processing.rs:278:49
    |
278 |                     funding_sats_read.map(|sats| Some(sats.0)).ok_or(DecodeError::InvalidValue)?;
    |                                                                ^^^^^ method not found in `Result<std::option::Option<u64>, lightning::ln::msgs::DecodeError>`
    |
note: the method `ok_or` exists on the type `std::option::Option<u64>`
help: use the `?` operator to extract the `std::option::Option<u64>` value, propagating a `Result::Err` value to the caller
    |
278 |                     funding_sats_read.map(|sats| Some(sats.0))?.ok_or(DecodeError::InvalidValue)?;
    |                                                               +

For more information about this error, try `rustc --explain E0599`.
error: could not compile `lightning-rapid-gossip-sync` due to previous error

[TheBlueMatt]

Ugh, sorry:

$ git diff-tree -U2 7ad6364ab 37756867a
diff --git a/lightning-rapid-gossip-sync/src/processing.rs b/lightning-rapid-gossip-sync/src/processing.rs
index 002807414b..8319506b57 100644
--- a/lightning-rapid-gossip-sync/src/processing.rs
+++ b/lightning-rapid-gossip-sync/src/processing.rs
@@ -274,7 +274,6 @@ where
 				let additional_data: Vec<u8> = Readable::read(read_cursor)?;
 				let mut cursor = &additional_data[..];
-				let funding_sats_read: Result<BigSize, _> = Readable::read(&mut cursor);
-				funding_sats =
-					funding_sats_read.map(|sats| Some(sats.0)).ok_or(DecodeError::InvalidValue)?;
+				let funding_sats_read: BigSize = Readable::read(&mut cursor)?;
+				funding_sats = Some(funding_sats_read.0);
 				if !cursor.is_empty() {
 					log_gossip!(