chore(node): use one key pair for both TLS AND near access key

[DSharifi]

Proof of concept PR to

• merge the two key pairs generated on the ndoe, TLS and Account key, into one key: https://nearone.slack.com/archives/C07UW93JVQ8/p1759043700318139
• removeNodeIdstruct and its usage as a unique identifier for attestations, and only rely on the nodes' public keys as identifiers as they are assumed to be unique:

Remaining work:

• Pytests and integration tests will also have to be updated and simplified to not rely on NodeId struct.
• Pytests must be simplified to only fetch a single key from the web endpoint.