rpc-v2: Change method name for provided events (#1593)

This PR changes the method name of the subscription that provides
JSON-RPC notifications (ie subscription events).
This brings the raw JSON response in sync with the rpc-spec-v2 format.

Changes:
- `chainHead_unstable_follow` to `chainHead_unstable_followEvent`
[spec/chainHead](https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/chainHead_unstable_follow.md#notifications-format)
- `transaction_unstable_submitAndWatch` to
`transaction_unstable_watchEvent`
[spec/tx](https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/transaction_unstable_submitAndWatch.md#notifications-format)

@paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <[email protected]>