Collect label and metadata from header

[trueleo]

Fixes #75 .

Description

This PR allows for metadata and tags to be fetched from headers instead of body of the event. This gives some flexibility in schema and multiple sources with different schema for available metadata can send logs to the same stream.

Client needs to use prefix x-p-meta- and x-p-tags- in headers to send key value pairs in a server post event. These headers are parsed and validated before being merged into body with p_metadata and p_tags field respectively

Changes

• introduced header_parsing::collect_labelled which parses headers key value pairs and gives a string formatted as "k1=v1,k2=v2"
• New Error type ParseHeaderError which implements ResponseError.
• Use above types in post event handler

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This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

[nitisht]

Looking great, some comments for readability

[nitisht]

👍🏽