VPN-4401: Cache JWT skew

[agentpietrucha]

Ticket

JIRA-VPN-4401

Description

Cache skew with remote time with TTL = 4 hours

Checklist:

• Changelog

Screenshots (optional, if UI related)

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This change is 

[pronebird]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @mmsinclair)

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a discussion (no related file):
Please cargo fmt the source.

[pronebird]

@pronebird reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed (commit messages unreviewed), 2 unresolved discussions (waiting on @agentpietrucha and @mmsinclair)

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nym-vpn-core/crates/nym-vpn-api-client/src/client.rs line 50 at r1 (raw file):

#[derive(Default, Debug)]
struct SkewState {

Nit: you may really benefit from holding Option<SkewState> which would eliminate the need for members to be Optional as both skew and expires_at are set together.

[pronebird]

@pronebird reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @agentpietrucha and @mmsinclair)

[neacsu]

@neacsu reviewed all commit messages.
Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @agentpietrucha, @mmsinclair, and @pronebird)

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nym-vpn-core/crates/nym-vpn-api-client/src/client.rs line 57 at r3 (raw file):

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum SkewStatus {
    Expired(),

nit: No need for it to be function, it can be just Expired

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nym-vpn-core/crates/nym-vpn-api-client/src/client.rs line 228 at r3 (raw file):

                tracing::debug!("Valid VPN API time skew");
                let local_time = OffsetDateTime::now_utc();
                let estimated_remote_time = local_time - skew;

This subtraction is dependent on the fact that the skew is computed in a certain way inside local_time_ahead_skew from what I can tell. Although it seems to be correct at this moment, it's somewhat code duplicating and I think it should be refactored and implementation details be kept in one place (maybe inside VpnApiTime methods)

[agentpietrucha]

@neacsu I'll reference your latest comments in #3921