Add `*.otlp.headers_list` property to support `OTEL_EXPORTER_OTLP_{SIGNAL}_HEADERS`

[jack-berg]

Similar to the support for .resource.attributes_list added #106 to support OTEL_RESOURCE_ATTRIBUTES, we should add *.otlp.headers_list to support OTEL_EXPORTER_OTLP_{SIGNAL}_HEADERS.

Yes I agree. I punted on that in #106 to minimize the scope of the PR, but definitely think we should add support for *.otlp.headers_list to allow compatibility with the OTEL_EXPORTER_OTLP_{SIGNAL}_HEADERS env vars.

Originally posted by @jack-berg in #115 (comment)