Add parser for pcap index

[cristinaleonr]

• Adding parser for pcap datatype
• Adding pcap datatype constants

Sample output

{
  "ID":"ndt-4c6fb_1625899199_00000000013A461F",
  "Parser":  {
    "Version":"local development",
    "Time":"2021-07-25T17:01:08.974024413Z",
    "ArchiveURL":"gs://archive-measurement-lab/ndt/pcap/2021/07/22/20210722T000107.470279Z-pcap-mlab1-dfw05-ndt.tgz",
    "Filename":"2021/07/22/ndt-4c6fb_1625899199_00000000013A461F.pcap.gz",
    "Priority":0,
    "GitCommit":"uninitialized"
  },
  "Date":"2021-07-22"
}

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 6583

• 53 of 71 (74.65%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 63.052%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/parser.go	0	2	0.0%
parser/pcap.go	53	69	76.81%
<!--	Total:	53	71	74.65%	-->

Totals
Change from base Build 6577:	0.1%
Covered Lines:	3570
Relevant Lines:	5662

---

💛 - Coveralls