Add P2A questions in Brave Ads

[moritzhaller]

Add p2a questions in the service layer via weekly storage

• Add total ad impressions question
• Add total ad opportunities question
• Add ad impressions per top level segment questions
• Add ad opportunity per top level segment questions

report on the following top level segments:
architecture, arts & entertainment, automotive, business, careers, cell phones, crypto, education, family & parenting, fashion, folklore, food & drink, gaming, health & fitness, history, hobbies & interests, home, law, military, other, personal finance, pets, real estate, science, sports, technology & computing, travel, weather and untargeted

[btlechowski]

Verification passed on

Brave	1.18.69 Chromium: 87.0.4280.88 (Official Build) (64-bit)
Revision	89e2380a3e36c3464b5dd1302349b1382549290d-refs/branch-heads/4280@{#1761}
OS	Windows 7 Service Pack 1 (Build 7601.24544)

Verified test plan from brave/brave-core#6811

Case 1: Untargeted ad
Verified logs for:

[9188:1176:1208/121045.537:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.TotalAdOpportunities has bucket count 9
[9188:1176:1208/121045.550:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.AdImpressionsPerSegment.untargeted has bucket count 9
[9188:1176:1208/121045.550:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.TotalAdImpressions has bucket count 9

Case 2: Targeted Ad
Verified logs for:

[7152:9020:1208/123107.490:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.AdOpportunitiesPerSegment.technologycomputing has bucket count 9
[7152:9020:1208/123107.490:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.AdOpportunitiesPerSegment.personalfinance has bucket count 9
[7152:9020:1208/123107.490:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.AdOpportunitiesPerSegment.fooddrink has bucket count 9
[7152:9020:1208/123107.490:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.TotalAdOpportunities has bucket count 9

Case 3: Disable P2A
Turn off ads

[12928:15952:1208/124431.386:VERBOSE1:ads_service_impl.cc(1657)] P2A histograms suspended
[12928:15952:1208/124431.386:VERBOSE1:ads_service_impl.cc(672)] Shutting down ads
[12928:15952:1208/124431.386:VERBOSE2:brave_p3a_service.cc(439)] BraveP3AService::OnHistogramChanged: histogram_name = Brave.P2A.TotalAdOpportunities Sample = 2147483646 bucket = 2147483646

Case 4: Re-enable Ads and make sure P2A/P3A works as expected

[4856:3048:1208/125046.694:VERBOSE1:ads_service_impl.cc(1657)] P2A histograms suspended
[4856:3048:1208/125046.694:VERBOSE1:ads_service_impl.cc(672)] Shutting down ads
[4856:3048:1208/125449.086:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.AdImpressionsPerSegment.untargeted has bucket count 9
[4856:3048:1208/125449.086:VERBOSE2:brave_p3a_service.cc(424)] P2A metric Brave.P2A.TotalAdImpressions has bucket count 9