Change Regs.COPPA, Banner.TopFrame, Video.MinBitRate to pointer types

[sujanchalla0510]

Fixes #13.

Summary

Regs.COPPA, Banner.TopFrame, and Video.MinBitRate were typed as
plain int8/int64 with json:"...,omitempty". encoding/json's
omitempty treats a value type's zero value as empty, so explicitly
setting one of these fields to a meaningful 0 was indistinguishable,
once marshaled, from never having set the field at all:

• Regs.COPPA: 0 means "not subject to COPPA" — a real,
  compliance-relevant signal, not "unknown".
• Banner.TopFrame: 0 means "not in the top frame".
• Video.MinBitRate: a real 0 Kbps floor is a valid value, per the
  spec.

All three were silently dropped from the marshaled JSON when set to
0, so a sender explicitly signaling "no"/"0" produced wire output
identical to never having set the attribute at all.

Fix

This repo already uses pointer types for the identical "0/1, omission
means Unknown" semantic on sibling fields in the same structs —
Regs.GDPR, Banner.Vcm, and Device.DNT — so this brings COPPA,
TopFrame, and MinBitRate in line with that existing, established
pattern rather than introducing a new one. Uses the existing
Int8Ptr/Int64Ptr helpers from ptr.go.

Breaking change

This is a source-breaking change for any caller directly assigning
these three fields as value types (e.g. banner.TopFrame = 1 becomes
banner.TopFrame = openrtb2.Int8Ptr(1)). Per the README this repo
follows semver with major-version releases for breaking changes (the
module is already at v20), so I'm assuming that's the expected path
here — happy to adjust if you'd prefer a different approach.

Test plan

• Added zero_value_fields_test.go, which marshals each field set
  explicitly to 0 and asserts it survives in the output. Verified
  these tests fail to even compile against the old value types
  (proving the fix is necessary), and pass against the fix.
• go build ./..., go vet ./..., go test ./... all pass.
• gofmt clean.