[pkg/pdatatest] Simplify creating test data for profiles support

[rockdaboot]

Fixes open-telemetry/opentelemetry-collector#12554

Currently, it is pretty tedious and error-prone to generate valid test profiles.

This PR adds Go structs and transform functions for profiles types that allow creating valid test cases in a human readable way. It also takes care for some semantics that make a profile valid, for example that the first entry in the string table is the empty string.

Here is an example from profiles_test.go

Before (this produces an invalid profile, btw)

	p := pprofile.NewProfiles()
	rl := p.ResourceProfiles().AppendEmpty()
	rl.Resource().Attributes().PutStr("key1", "value1")
	l := rl.ScopeProfiles().AppendEmpty().Profiles().AppendEmpty()
	attr := l.AttributeTable().AppendEmpty()
	attr.SetKey("scope-attr1")
	attr.Value().SetStr("value1")
	l.AttributeIndices().Append(0)
	l.SetProfileID(pprofile.NewProfileIDEmpty())
	rl2 := p.ResourceProfiles().AppendEmpty()
	rl2.Resource().Attributes().PutStr("key2", "value2")
	l2 := rl2.ScopeProfiles().AppendEmpty().Profiles().AppendEmpty()
	attr = l2.AttributeTable().AppendEmpty()
	attr.SetKey("scope-attr2")
	attr.Value().SetStr("value2")
	l2.AttributeIndices().Append(0)
	l2.SetProfileID(pprofile.NewProfileIDEmpty())
	return p

After

	p := basicProfiles()
	p.ResourceProfiles = append(p.ResourceProfiles, ResourceProfile{
		Resource: Resource{
			Attributes: []Attribute{{"key2", "value2"}},
		},
		ScopeProfiles: []ScopeProfile{
			{
				Profile: []Profile{
					{
						Attributes: []Attribute{{"scope-attr2", "value2"}},
					},
				},
			},
		},
	})
	return p.Transform()

where basicProfiles() is a helper function that creates a basic test Profiles.

Planned a follow-up PR with a validator for profiles (working locally already).