Implement "reset cryptographic identity" flow in new Encryption settings

[richvdh]

Part of #26468

This issue tracks the implementation of the "Reset cryptographic identity" flow in our designs for the new "Encryption" settings tab.

Our designs for the "Advanced" section of the Encryption settings tab includes a "Reset cryptographic identity" link (Figma design). The link takes you to a confirmation screen (Figma).

Clicking "Continue" on the confirmation screen should:

• Disable backing up room keys and delete the active backup
• Remove the account data entries for secret storage (ie, disable recovery)
• Create a new cross-signing identity and publish the public keys
• Assuming backup was previously enabled, re-enable it with a new keypair.

In other words, it should do the same as matrix_sdk::encryption::recovery::Recovery::reset_identity in the Matrix Rust SDK.

[Note that it does more than "reset identity". The UI refers to it as "Reset cryptographic identity" for consistency with EX, but we should avoid propagating the confusion in new code.]

After the reset, we now have no recovery, so we return to the basic Encryption tab which prompts you to "Set up Recovery".

We have agreed to add a new method CryptoApi.resetEncryption to matrix-js-sdk which will implement the reset mechanics.

Internal discussion

[richvdh]

matrix-org/matrix-rust-sdk#4502 is about a bug in reset_identity that we should avoid reproducing