What's changing in this PR?
This PR introduces the initial changes for "(2) Standardizing all tracer libraries to using the current application name as a default for existing services (service naming)". The major changes in this revision are:

Create a new concept, 'span attribute schema,' which controls the service naming behavior and control it with a new environment variable DD_TRACE_SPAN_ATTRIBUTE_SCHEMA.
Create 'v0' (existing behavior) and 'v1' (new behavior) schemas and behaviors:
v0 (Default) - The existing behavior
v1 - Default integration service names are now set to the value of
$DD_SERVICE (config().service) instead of the prior default values

Specifically for the GRPC integration. Other PRs are being merged for the other integrations.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed.
• Documentation is included (in-code, generated user docs, public corp docs).
• OPTIONAL: PR description includes explicit acknowledgement of the performance implications of the change as reported in the benchmarks PR comment.

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.