Graylog2-server: Allow converters on Grok extractors

[bpbp-boop]

It would be handy to allow converters on Grok patterns.

For example, with the following pattern:

POSTFIX_PIPE_DELIVERED %{POSTFIX_QUEUEID:postfix_queueid}: %{GREEDYDATA:postfix_keyvalue_data} \(delivered via %{WORD:postfix_pipe_service} service\)

The field postfix_keyvalue_data will always contain key=value data. To extract this I currently need to create a copy extractor with a key/value converter on the resulting postfix_keyvalue_data field, duplicating a lot of information

[drewmmiranda]

Same for forcing the numeric type for use with statistics and charts.

[manishjhawar]

+1, especially for casting to scalar types such as numbers for metrics

[jensschulze]

+1

[joschi]

The grok patterns already support type hints, for example %{NUMBER:num:int} will convert the match named "num" into an integer.

Currently the following types are supported:

• byte
• boolean
• short
• int
• long
• float
• double
• date
• datetime
• string