Maven Artifacts: Add publish date to index

[bhermann]

Is your feature request related to a problem? Please describe.
Currently, it is not possible to find out when an artifact was published.

Describe the solution you'd like
Add the publish date based on the last modified date from the HTTP header.
Maybe consider other fields as well.

Describe alternatives you've considered
none

Additional context

> curl --head http://repo1.maven.org/maven2/abbot/abbot/1.4.0/abbot-1.4.0.pom
HTTP/1.1 200 OK
ETag: "643667fad16f969edec49157b0cc5480"
Content-Type: text/xml
Last-Modified: Tue, 22 Sep 2015 16:03:12 GMT
X-Checksum-MD5: 643667fad16f969edec49157b0cc5480
X-Checksum-SHA1: c7d371c9363c13073aa5763d7ceb196f78c2d5cb
Via: 1.1 varnish
Content-Length: 1292
Accept-Ranges: bytes
Date: Wed, 22 May 2019 20:15:55 GMT
Via: 1.1 varnish
Age: 1678759
Connection: keep-alive
X-Served-By: cache-iad2144-IAD, cache-fra19179-FRA
X-Cache: HIT, HIT
X-Cache-Hits: 1, 1
X-Timer: S1558556155.144471,VS0,VE0