in_s3: enable multiline parsing

### Is your feature request related to a problem? Please describe.

I noticed me and many users are wondering to be able to recognize logs that are on many lines.

Currently any plugin seemes to satisfy the thing, because:
- the fluentd multiline parsers applys on in_tail input only
- the [fluent-plugin-concat](https://github.com/fluent-plugins-nursery/fluent-plugin-concat) plugin works only if the numeber of lines is well defined

### Describe the solution you'd like

I propose to embed the multiline parser in the [fluent-plugin-s3](https://github.com/fluent/fluent-plugin-s3) as it works for in_tail input

### Describe alternatives you've considered

I tried the flluent-plugin-concat plugin, but it ignore the multiline_start_regexp if it is not specified a fixed number of lines

I tried also applying the multiline parser with S3 source, but I got error

### Additional context

_No response_