feat: Add metadata fields to core entities (FE)

[novakzaballa]

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Changes

PR Re-created (ORIGINAL: #2861)
FE changes:

• Add and create metadata to Environments, segments, and Flags
• The feature was wrapped inside the 'enable_metadata' flag

BE changes:

• feat: Add metadata fields to core entities (API) #3315

How did you test this code?

• Run feat/add-metadata-fields-to-core-entities-api
• Go to Project settings
• Click the Metadata tab.
• Create Metadata.
• Click on the metadata created, And select the entities you want to have metadata.

[matthewelwell]

Approving since the one comment I had on the frontend changes has been resolved. The rest are API related, and the API code was moved to a separate PR.