Implement _get_lastbday
Orthogonal to other PRs.

Implement `_get_lastbday` mirroring `_get_firstbday`.

Add a dummy argument to get_firstbday so that the signatures match.  Following this and #18218, we'll be able to define e.g.

```
class BQuarterBegin(...):
    _get_bday = _get_firstbday

class BMonthEnd(...):
    _get_bday = _get_lastbday
```

etc.  From there we can get rid of _bunch_ of duplicated logic.


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=h1) Report
> Merging [#18234](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/96a527434a6138b291c1b4a782bd2793cff51f74?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18234/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18234      +/-   ##
==========================================
- Coverage   91.43%   91.39%   -0.05%     
==========================================
  Files         163      163              
  Lines       50091    50092       +1     
==========================================
- Hits        45800    45780      -20     
- Misses       4291     4312      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.2% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.36% <7.14%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.11% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=footer). Last update [96a5274...c64adad](https://codecov.io/gh/pandas-dev/pandas/pull/18234?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Too many open PRs, need to exercise restraint. Closing.