[feature] Add config option to expose custom emojis without auth (#4233)

# Description

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>
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Does as it says on the tin! Should make things a bit easier for clients that don't provide an access token to the custom emojis endpoint.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/2430

## Checklist

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- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
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- [x] I/we have performed a self-review of added code.
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- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
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Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4233
Reviewed-by: Daenney <daenney@noreply.codeberg.org>
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

docs/api/swagger.yaml

@@ -3752,6 +3752,7 @@ info:
         read:applications: grants read access to user-managed applications
         read:blocks: grants read access to blocks
         read:bookmarks: grants read access to bookmarks
+        read:custom_emojis: grants read access to custom emojis
         read:favourites: grants read access to accounts
         read:filters: grants read access to filters
         read:follows: grants read access to follows
@@ -8126,6 +8127,7 @@ paths:
                 - conversations
     /api/v1/custom_emojis:
         get:
+            description: If the instance config setting `instance-expose-custom-emojis` is `true` then authentication is not required.
             operationId: customEmojisGet
             produces:
                 - application/json
@@ -8143,7 +8145,8 @@ paths:
                 "500":
                     description: internal server error
             security:
-                - OAuth2 Bearer: []
+                - OAuth2 Bearer:
+                    - read:custom_emojis
             summary: Get an array of custom emojis available on the instance.
             tags:
                 - custom_emojis
@@ -13588,6 +13591,7 @@ securityDefinitions:
             read:applications: grants read access to user-managed applications
             read:blocks: grants read access to blocks
             read:bookmarks: grants read access to bookmarks
+            read:custom_emojis: grants read access to custom emojis
             read:favourites: grants read access to accounts
             read:filters: grants read access to filters
             read:follows: grants read access to follows