feat: update BroadcastMessage to avoid alloc

[szpnygo]

only encode once in BroadcastMessage to avoid alloc objects

[codecov]

Codecov Report

Merging #40 (b1ae995) into main (c2b5f15) will decrease coverage by 1.71%.
The diff coverage is 17.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   60.95%   59.23%   -1.72%     
==========================================
  Files          68       67       -1     
  Lines        2876     2971      +95     
==========================================
+ Hits         1753     1760       +7     
- Misses        981     1065      +84     
- Partials      142      146       +4     

Impacted Files	Coverage Δ
app.go	72.72% <ø> (-1.81%)	⬇️
e2e/performance/main.go	0.00% <0.00%> (ø)
static.go	46.51% <60.86%> (ø)
agent/agent.go	51.47% <75.00%> (+0.71%)	⬆️
builder.go	100.00% <100.00%> (ø)
session/session.go	75.00% <100.00%> (+1.19%)	⬆️
codec/codec.go	80.00% <0.00%> (-1.82%)	⬇️