requesthandler: Add scene item blend method requests

[Agash]

Description

Adds GetSceneItemBlendMethod and SetSceneItemBlendMethod, mirroring the existing GetSceneItemBlendMode / SetSceneItemBlendMode pair, and adds a sceneItemBlendMethod field to the scene item list array next to the existing sceneItemBlendMode.

Valid values are OBS_BLEND_METHOD_DEFAULT and OBS_BLEND_METHOD_SRGB_OFF, converted via a new nlohmann type converter for obs_blending_method.

Motivation and Context

Closes #1341.

Browser sources used as overlays need their blending method set to sRGB Off for transparency to composite correctly, otherwise semi-transparent pixels show colour fringing and broken blending. SetSceneItemBlendMode only covers the blending mode, so this was the one step of overlay setup that could not be automated and had to be done by hand in the UI.

How Has This Been Tested?

Built OBS Studio from source with this branch as the plugins/obs-websocket submodule, then drove the requests over the protocol against the running instance:

• GetSceneItemBlendMethod returns OBS_BLEND_METHOD_DEFAULT on a fresh scene item
• SetSceneItemBlendMethod to OBS_BLEND_METHOD_SRGB_OFF and back to OBS_BLEND_METHOD_DEFAULT, each confirmed by a readback
• an unknown value is rejected with 400 InvalidRequestField, a non-string with 401, a missing field with 300, and an unknown sceneItemId with 600
• the same set/get pair through a request batch
• GetSceneItemList reports the updated sceneItemBlendMethod
• after closing OBS, the scene collection on disk records blend_method: srgb_off for the item

Tested OS(s): Windows 11

Types of changes

• New request/event (non-breaking)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.