[exporter/prometheusremotewriteexporter] fix config key collision: un-squash client config (issue 37332)

[vigr]

Description

Prometheus remote write HTTP client config is squashed into the exporter config hence causing a collision for timeout key.
This PR fixes the issue (#37332) by putting the HTTP config under its own key prw_client (unsquashed) in a backward-compatible manner.

Link to tracking issue

Fixes #37332

Testing

New test:

• Test_NewPRWExporter_ClientConfig

Additional case for test:

• TestLoadConfig

Documentation

My intention is to put the HTTP client config under its own key prw_client such that:

• it's backward-compatible: prior configs will continue working
• if new prw_client config section is specified, it takes precedence over the squashed version
• default options for the prw_client are preserved in a single place: see setDefaultsPRWClientConfig function
• the squashed version (Config.ClientConfig) is deprecated in favor of the new format

[linux-foundation-easycla]

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