OIDC: increase length of generated nonce parameter from 30->32 chars

[anoadragon453]

This PR slightly increases the character length (and thus the entropy) of the nonce value (from the default 30 characters -> 32 characters) that Synapse generates when performing an OIDC login.

This is so that Synapse's OIDC implementation complies with the relevant portion of the TI-Messenger spec - which aims to standardise Matrix as the communication layer for healthcare in Germany.

The spec requires (translation via Google Translate):

The Authorization Server service MUST generate a nonce (random value) at runtime in accordance with [ RFC7636 # section-4.1 ]. The nonce MUST contain an entropy of at least 43 and a maximum of 128 characters.

We clarified with Gematik (the German government body in charge of writing this spec) what they meant exactly by "an entropy of at least 43". They replied that they meant to write "43 characters".

When sent to the Identity Provider, the nonce value is encoded using base64url. A 32 character string encoded to base64url results in a 43 character length string. Thus, this change aligns Synapse's generated nonce value with the spec.

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While this change is intended to satisfy an external spec, the change only increases security (while only adding three extra bytes to a request). The matrix spec does not cover this flow, instead (rightly) offloading it to RFC7636. That RFC only demands a 43 character length for the code_verifier parameter, which Synapse does comply with. Whereas Gematik have opted to adapt that length requirement to the nonce parameter as well.