Add PaxosLease package and tests

[ljluestc]

This PR implements the PaxosLease mechanism for PD leader election, as proposed in issue #1677.
Currently, PD relies heavily on etcd for leader election and TSO timestamp allocation guarantees. This creates a strong dependency on etcd's availability and write latency.

PaxosLease allows PD members to elect a leader using a disk-less Paxos implementation that maintains lease state in memory across a quorum of nodes. This reduces the dependency on etcd writes for lease renewal and allows for potentially faster failure detection and TSO service recovery.

What is changed and how does it work?

I have introduced a new package pkg/paxoslease that contains a standalone implementation of the PaxosLease algorithm.

Key components:

• Paxos struct: Implements the Paxos state machine (Proposer, Acceptor, Learner).
• Lease struct: Represents the lease value (Owner, Duration, StartTime).
• Transport interface: Decouples the consensus logic from the networking layer, allowing integration with PD's existing communication channels.
• Mockable Time: The implementation uses a swappable time source to ensure deterministic testing of lease expiry and renewal.

Logic Flow:

1. Campaign: A node initiates a Paxos Prepare phase.
2. Promise: Nodes respond with Promise and any previously accepted values.
3. Accept: The proposer selects a value (respecting existing valid leases) and broadcasts Accept.
4. Learn: Once a quorum accepts a value, it is considered chosen, and the node becomes the leader for the lease duration.
5. Renewal: The current leader can preemptively renew the lease before it expires to maintain continuous leadership.

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• f8ac984 Add PaxosLease package and tests
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