chore(dataobj): backfill sparse rows using EncodeN

[ashwanthgoli]

What this PR does / why we need it:

We currently backfill old rows with nulls if they have no value for a given column. We do this by calling Encode(value) N times, but for sparse columns we have to iterate over very large numbers to perform the backfill. This PR adds EncodeN(value, n) method to bitmapEncoder to allow appending a value N times - this runs in constant time unlike the existing approach which takes O(n)

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/v3/pkg/dataobj/internal/dataset
cpu: Apple M1 Pro
Benchmark_bitmap_EncodeN/Encode_1000_times-8              227778              5262 ns/op
Benchmark_bitmap_EncodeN/EncodeN_1000_times-8           83439080                14.29 ns/op
Benchmark_bitmap_EncodeN/Encode_10000_times-8              22904             52238 ns/op
Benchmark_bitmap_EncodeN/EncodeN_10000_times-8          74150934                16.06 ns/op
Benchmark_bitmap_EncodeN/Encode_100000_times-8              2307            526224 ns/op
Benchmark_bitmap_EncodeN/EncodeN_100000_times-8         72333274                16.34 ns/op
Benchmark_bitmap_EncodeN/Encode_1000000_times-8              226           5229343 ns/op
Benchmark_bitmap_EncodeN/EncodeN_1000000_times-8        77011526                15.92 ns/op
PASS

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

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• Documentation added
• Tests updated
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