Extended jinja template to generate template-type semantic attributes.

[AlexanderWert]

With open-telemetry/build-tools#186, template-type attributes (such as http.request.header.<key>) can be defined in semantic convention model yaml files. With that, these template attributes can be used in code generation for the semantic conventions.

This PR adds a new class AttributeKeyTemplate for this kind of attribute keys and extends the code generation to cover attributes like http.request.header.<key>, db.elasticsearch.path_parts.<key>, rpc.grpc.request.metadata.<key>, etc.

With this change utility classes like CapturedHttpHeadersUtil in the java-instrumentation repo can be replaced with instances (and corresponding constants) of the more generic AttributeKeyTemplate class.

[AlexanderWert]

@open-telemetry/java-approvers PTAL, I think this would allow to replace some custom helpers in the agent with generated constants for this type of attributes

[trask]

cc @breedx-splk @mateuszrzeszutek to review new public API surface if you have a chance

[breedx-splk]

This looks good to me. Just have the one idea around the key cache being static, but otherwise seems like a nice addition.

[AlexanderWert]

@jack-berg , @trask , @mateuszrzeszutek , @breedx-splk

Based on the discussions above I reverted the implementation to use a final , inline-initialized map (with initial size 1) again.
PTA(nother)L, Thanks!

[mateuszrzeszutek]

LGTM, I left one more minor JavaDoc comment.
Thanks!

[AlexanderWert]

OK, then I think it's ready to be merged.

[jack-berg]

Couple nits and a question about whether the methods to create AttributeKeyTemplate should be public to allow users to create their own templates. None of the comments should block this PR.