feat: add attribute definition for fallback support

[aaron-steinfeld]

Description

This allows fallback support, where we can propose alternative definitions of an attribute that will be tried in order. For example, if we wanted to get the version of the tracer we could define an attribute with a definition like below, first trying the hypertrace version attribute, and if missing using the otel spec defined one. Changes to support evaluating this will be added to the attribute reader.

definition: {
  first_value_present: {
    definitions: [
      {
        source_path: hypertrace.module.version
      },
      {
        source_path: telemetry.sdk.version
      }
    ]
  }
}

Testing

API addition only, but verified it functionally in an e2e setup.

[codecov]

Codecov Report

Merging #84 (f0a64a9) into main (c89a6b2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #84   +/-   ##
=========================================
  Coverage     80.71%   80.71%           
  Complexity      222      222           
=========================================
  Files            27       27           
  Lines           752      752           
  Branches         56       56           
=========================================
  Hits            607      607           
  Misses           98       98           
  Partials         47       47           

Flag	Coverage Δ	Complexity Δ
integration	80.71% <ø> (ø)	0.00 <ø> (ø)
unit	67.77% <ø> (ø)	0.00 <ø> (ø)

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[github-actions]

Unit Test Results

20 files  ±0  20 suites  ±0   11s ⏱️ -1s
75 tests ±0  75 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 45b88ce. ± Comparison against base commit c89a6b2.