Add sourcemap upload feature to webpack plugin

[mvirgil]

Tested locally with olly RUM UI project

Description

Add "upload" functionality to the webpack plugin.

When this webpack plugin is enabled, it will automatically upload source map assets
to the backend once the build is finished.

Example output in the webpack logs

<i> [OllyWebPlugin] Uploading 24 source maps to http://localhost:8088/v1/sourcemaps/id/{id}
<w> [OllyWebPlugin] 413 Request Entity Too Large
<w> [OllyWebPlugin] Unable to upload rumModule.cf999b4569620b6898e5.js.map
<i> [OllyWebPlugin] Successfully uploaded 23 source map(s)
<e> [OllyWebPlugin] Failed to upload 1 source map(s)
[OllyWebPlugin] Uploading finished

Type of change

Delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Chore or internal change (changes not visible to the consumers of the package)

How has this been tested?

Delete options that are not relevant.

• Manual testing

Manually tested with the O11y RUM webpack build.
Separate JIRA task for handling integration testing
Planning to remove some new util function soon (for computing source map ID) after
the inject command is updated, so omitted unit test for those as well.

[shumone]

Looks good 👍