## Description
Since https://github.com/odoo/odoo/pull/76734, the new field `alias_full_name` is used in addition to `alias_name` to search on `mail.alias` and route emails in `message_route`. It's also used as a search criteria in `_search_alias_email` on `mail.alias.mixin. optional`. The issue is that the table `mail.alias` can grow quite large, and only the old field `alias_name` has an index on it, which may not be selective enough, forcing Seq.Scans on the table with possibly millions of records. The impact is noticeable on `message_route` which is a hot path for processing incoming emails.

Adding an index on `alias_full_name` allows PostgreSQL to do an bitmap OR scan on the two indexes, considerably speeding up search criteria that are a disjunction between `alias_name` and `alias_full_name`.

## Benchmark
For domain
```python
[
    '&',
    ('alias_model_id', '!=', reply_model_id),
    '|',
    ('alias_full_name', 'in', email_to_list),
    '&', ('alias_name', 'in', email_to_localparts), ('alias_incoming_local', '=', True),
]
```
with test arguments, on a `mail.alias` table containing over 2M records.

|           | Before | After |
|-----------|--------|-------|
| Timing    | 696ms  | 1ms   |
| Buffer IO | 790MB  | 48KB  |

Specially impactful as those gains needs to be multiplied by the frequency of the searches.

## Reference
task-3724844

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