[processor/lookupprocessor] Add skeleton for external lookup enrichment

[jsvd]

Description

Adds the initial skeleton for a lookup processor that performs external lookups to enrich telemetry signals:

• Processor factory with logs support (traces/metrics planned)
• Source abstraction with factory pattern for extensibility
• Built-in noop source for testing
• Cache wrapper utility for source implementations
• WithSources option for custom source registration

Link to tracking issue

First part of #41816

Testing

Zero testing so far.
I'll add processor and lookupsource factory testing in a separate PR.
More tests will be added when the processor implementation is added.

Documentation

For now most of it is in README.md

[jagan2221]

Hi @jsvd, Just some suggestions(Pls ignore if you already planned these for next phase)

1. Since this lookup processor is going to be extended to a wide variety of lookups, would it makes sense to add "context (resource/record)" parameter as well to the settings? since LookupFunc interface definition just accepts the key(string) to perform lookup.
2. Once source.lookup is done, how are going to add the resolved value to attributes?
   Example: (Defined in the component proposal desc) we have this section which is common for the processor irrespective of the type of lookup.
   attributes:
   • key: client.hostname
     from_attribute: client.ip
     default: "unknown"
     action: upsert
     context: log

[jsvd]

@jagan2221 this PR is mostly just the skeleton, doesn't yet have the processor implementation itself.
I have a second PR incoming, you can see the code in my personal repo here jsvd#3, it is a single commit on top of the commit from this PR.

The README shows how the processor actually works with a source: https://github.com/jsvd/opentelemetry-collector-contrib/blob/adc7da937d1b65e1272296a4b0081d23fa6e83d8/processor/lookupprocessor/README.md#configuration

processors:
  lookup:
    source:
      type: yaml
      path: /etc/otel/mappings.yaml
    attributes:
      - key: user.name
        from_attribute: user.id
        default: "Unknown User"
        action: upsert
        context: record

Does this satisfy your needs?

[jagan2221]

Looks good then, thanks!