[Refactor][EAGLE] 7/N Merged PCP and disable_padded interface (#6811)

### What this PR does / why we need it?
[Refactor][EAGLE] 7/N Merged PCP and disable_padded interface into
eagle_proposer.py

This pull request significantly refactors the speculative decoding
mechanism by merging Parallel Context Processing (PCP) and Multi-Token
Prediction (MTP) functionalities directly into the eagle_proposer.py.
The changes aim to enhance the efficiency and correctness of distributed
speculative decoding, particularly by enabling the Eagle feature to work
seamlessly with the disable_padded interface. This involves detailed
adjustments to attention metadata, input/output processing, and state
management to ensure proper operation in parallel environments.

1. The PCP and MTP features are migrated to the eagle_proposer.py
2. The Eagle and PCP features are integrated
3. Enable the eagle feature to use the disable_padded interface

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Tests and UT

- vLLM version: v0.15.0
- vLLM main:
83b47f67b1

---------

Signed-off-by: lilinsiman <lilinsiman@gmail.com>

vllm_ascend/attention/attention_v1.py

@@ -159,6 +159,7 @@ class AscendMetadata:
     num_decode_tokens: int = 0
     num_prefills: int = 0
     num_decodes: int = 0
+    num_decodes_flatten: int = 0
 
     # The sequence length per sequence. Sequence length means the computed
     # tokens + new tokens (is None if it is a decoding).