[Perf] Make Brave Extension disabled by default to optimize memory usage

[atuchin-m]

Description

Currently, we load Brave Extension on startup and it's background page stays persistent. It consumes about 40MB of memory (mac-arm64) and has some CPU overhead.

Features depending on Brave Extension:

1. content picker: Don't inject content picker on pages by default #37841
2. Greaselion (despite it creates a number of supplementary extensions, an investigation is needed)
3. WDP (when enabled)

The goal is to not load Brave Extension when WDP is disabled. 1 & 2 should be reworked.
Or even rename it to WDP extension.

Related issues:
#21366
#35645

[atuchin-m]

The plan for content picker feature:

1. Stop injecting content_picker by default in every frame: Don't inject content picker on pages by default #37841
2. Make a native menu instead an extension-based;
3. Stop using iframe for content picker;
4. Replace extension-based inject to C++ inject.

[atuchin-m]

for Greaselion: here is PoC to remove chrome.tabs and chrome.webRequest API: brave/brave-site-specific-scripts#119. This approach allows to refactor Greaselion to not use a persistent background page

[atuchin-m]

The current status of content picker changes:

1. [done] remove inject by default: Inject content_picker dynamically brave-core#25330
2. [done] content Picker refactoring into a single script, stop using the backround page: Content Picker refactoring into a single script brave-core#25358
3. [in-progress] native menu + native inject via CosmeticFiltersJSHandler: Content picker native inject brave-core#25411

[atuchin-m]

A Greaselion related issues:

1. Disable Greaselion service if Rewards isn't enabled #40872
2. Remove Greaselion after Rewards stop using it #40873

[atuchin-m]

Test plan (for both this issue and #40872)

Case 1: Rewards and WDP are disabled:

1. Restart browser
2. Open a new tab and navigate to a page that uses Greaselion (e.g. https://x.com/bravelaurenwags on staging)
3. Verify that the creator checkmark does not appear on the Rewards icon
4. Verify there is no Brave Extension process in menu => More tools => Task manager
5. Verify there is no extension with name greaselion- on brave://extensions-internals/

Case 2: Rewards ON:

1. Open the browser with a clean profile
2. Enable rewards
3. Open a new tab and navigate to a page that uses Greaselion (e.g. https://x.com/bravelaurenwags on staging)
4. Verify that the creator checkmark appears on the Rewards icon
5. Verify there is Brave Extension process in menu => More tools => Task manager

Case 3: WDP ON:

1. Open the browser with a clean profile
2. Enable WDP
3. Verify there is Brave Extension process in menu => More tools => Task manager
4. check WDP basic functionality (ask @remusao if you need instructions)