Fix case-sensitive HTTP headers in W3C traces

[puffnfresh]

When a header did not match "traceparent" case-sensitively, a new trace ID would be generated and used, severing the tie to other services. This commit introduces a value type which normalises header name case, for comparison.

Before:

curl -v -H 'Traceparent: 00-aaaa...-bbbb...-01' http://<pod>:5003/api/validator/readyz
# traceparent: 00-cccc...-dddd...-03

curl -H 'traceparent: 00-aaaa...-bbbb...-01' http://<pod>:5003/api/validator/readyz
# traceparent: 00-aaaa...-bbbb...-01

After:

curl -v -H 'Traceparent: 00-aaaa...-bbbb...-01' http://<pod>:5003/api/validator/readyz
# traceparent: 00-aaaa...-bbbb...-01

curl -H 'traceparent: 00-aaaa...-bbbb...-01' http://<pod>:5003/api/validator/readyz
# traceparent: 00-aaaa...-bbbb...-01

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