Support Endpoint Override for CredentialsProviders

[waahm7]

Issue #, if available:
#257
awslabs/aws-crt-swift#311

Description of changes:
Support endpoint override for credentials providers.

The order of resolution for configured endpoint is as follows:

1. The value provided by a service-specific environment variable, AWS_ENDPOINT_URL_<SERVICE>. <SERVICE> here is the uppercased version of corresponding service identifier in this official list.
2. The value provided by the global endpoint environment variable, AWS_ENDPOINT_URL.
3. The value provided by a service-specific parameter from a services definition section referenced in a profile in the shared configuration file. The name of the service must match the identifier in this official list. E.g.:

[profile profile-with-services]
services = dev
endpoint_url = http://localhost:5567

[services dev]
sts = 
  endpoint_url = https://play.min.io:9000

4. The value provided by the global parameter from a profile in the shared configuration file. E.g.:

[profile profile-with-services]
services = dev
endpoint_url = http://localhost:5567

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.95775% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.61%. Comparing base (b513db4) to head (dcb8868).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
source/credentials_provider_sts.c	85.71%	3 Missing ⚠️
source/credentials_utils.c	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
+ Coverage   80.50%   80.61%   +0.11%     
==========================================
  Files          33       33              
  Lines        6099     6140      +41     
==========================================
+ Hits         4910     4950      +40     
- Misses       1189     1190       +1     

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[graebm]

a few lines up, we can leak the region string if it's ""

[graebm]

the aws_credentials_provider_resolve_region_from_env() function has the same bug

[graebm]

should we just have an alternate get-env-var function that won't return empty strings?

[graebm]

oh man, if we're adding new get-env functions, we can fix the terrible API of the current one (bad because it did out-values to handle reporting OOM which we don't do anymore, and bad because it didn't just take char * which forces some users to allocate/cleanup a needless aws_string)

struct aws_string *aws_getenv_nonempty(struct aws_allocator *, const char *); // NULL if missing or ""
struct aws_string *aws_getenv_raw(struct aws_allocator *, const char *); // may be ""

[graebm]

I'm not proposing an options-struct because this is private

But if someone moves this out of private/ and is like "WHY SHOULD I CHANGE IT YOU ALREADY APPROVED THE EXISTING CODE"

then I can point at this comment here and say "NUH UH"

[graebm]

if something goes wrong here, we end up setting out_region but not out_endpoint?

is that failure? or is that something we expect to happen?

[waahm7]

Logically, it won't happen since if we resolve the region, we can construct the endpoint. But even if it does happen in the future, we have a fallback: We will check that out_endpoint is not set and then use the global STS endpoint.

[graebm]

code clarity: *out_region could be NULL at this point....
and I see that aws_credentials_provider_construct_endpoint() only requires region sometimes

if region isn't always required, add a comment here to that effect

but if it is always required, maybe early-out if s_resolve_region() returns NULL

[graebm]

there are a lot of paths where NULL is returned, but no aws_raise_error() happened. Is that OK for this helper function?

[waahm7]

Yes, NULL means that there was no endpoint_override configured.