Support custom regex for x-release-please-* annotations in extra-files

Currently, the [version updater](https://github.com/googleapis/release-please/blob/main/docs/customizing.md#updating-arbitrary-files) in extra-files allows us to specify an inline line to update (with `x-release-please-*`) or a block of lines (with `x-release-please-start-*`).

However, within the block of lines it replaces any string that looks like a version number.

I have a fenced code block that provides sample code for installing my library (Google-maps-iOS-Utils) alongside a related dependency (GoogleMaps). I want release-please to update the dependency for my library without updating the version number of the other dependency.

When I try marking the code block, release-please updates every version number in the code block.

**Input code**

```markdown

```ruby
use_frameworks!

target 'TARGET_NAME' do
pod 'GoogleMaps', '8.0.0'
pod 'Google-Maps-iOS-Utils', '4.2.2'
end
\```

```

**Output (see [automated commit](https://github.com/googlemaps/google-maps-ios-utils/pull/476/commits/07987a3332756a7c1ce79d003ef9574d5757e506#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5) in the Release PR)**

  use_frameworks!


  target 'TARGET_NAME' do

    pod 'GoogleMaps', '5.0.0'

    pod 'Google-Maps-iOS-Utils', '5.0.0'

  end

When I try marking the single line with an inline comment, release-please actually ignores that whole code block and updates *other* version numbers in other parts of the file.

**Input code**

```ruby
use_frameworks!

target 'TARGET_NAME' do
pod 'GoogleMaps', '9.0.0'
pod 'Google-Maps-iOS-Utils', '5.0.0' # x-release-please-version
end
```

**Output (see [automated commit](https://github.com/googlemaps/google-maps-ios-utils/pull/480/commits/b1483563f7dd946905ceca9829fb8d70c389a85c#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5) in the Release PR)**

_no change_

  use_frameworks!


  target 'TARGET_NAME' do

    pod 'GoogleMaps', '9.0.0'

    pod 'Google-Maps-iOS-Utils', '5.0.0' # x-release-please-version

  end

...
_changed without any annotation_

- github "googlemaps/google-maps-ios-utils" ~> 4.1.0

+ github "googlemaps/google-maps-ios-utils" ~> 6.0.0

## Proposal

- Support more fine-grained definition of the string pattern to replace by accepting a regex parameter instead of the default regex pattern that looks for any major.minor.patch version number string. Perhaps something like the [semantic-release-replace-plugin](https://github.com/jpoehnelt/semantic-release-replace-plugin) to accept a filename and a regex pattern. [Example usage](https://github.com/googlemaps/google-maps-ios-utils/blob/b56b39cec0e3d5938ddadbfeb4d3fec541ebbb5e/.releaserc#L6-L31) in a release config file before we migrated to release-please.

## Alternatives

- Perhaps the second case (inline annotation within a code block) is behaving in an unexpected way due to a bug, and fixing the bug will satisfy my needs by supporting inline annotations within a code block.
- Support specifying a line number(s) instead of relying on code comment to identify which lines to search in.