Add Avro schema builder

[hariso]

Description

Closes #52.

I've tried writing a few schemas, and the code is mostly verbose because of error handling. Namely, errors need to be handled for every field that is created. Creating some of the schemas can return an error too. The builder solves the issue by:

• accepting arguments needed to create a field
• creating the field internally and storing the error
• returning all the errors when a schema is built

Before

fInt, err := avro.NewField("int_field", avro.NewPrimitiveSchema(avro.Int, nil), avro.WithDefault(100))
if err != nil {
	// handle error
}
enumSchema, err := avro.NewEnumSchema("enum_schema", "enum_namespace", []string{"val1", "val2", "val3"})
if err != nil {
	// handle error
}
fEnum, err := avro.NewField("enum_field", enumSchema)
if err != nil {
	// handle error
}
schema, err := avro.NewRecordSchema("schema_name", "schema_namespace", []*avro.Field{fInt, fEnum})
if err != nil {
	// handle error
}
bytes, err := schema.MarshalJSON()
if err != nil {
	// handle error
}
fmt.Println(bytes)

After

enumSchema, err := avro.NewEnumSchema("enum_schema", "enum_namespace", []string{"val1", "val2", "val3"})
if err != nil {
	// handle error
}
bytes, err := NewBuilder("schema_name", "schema_namespace").
	AddField("int_field", avro.NewPrimitiveSchema(avro.Int, nil), avro.WithDefault(100)).
	AddField("enum_field", enumSchema).
	MarshalJSON()
if err != nil {
	// handle error
}
prettyPrint(bytes)

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[lovromazgon]

Looks like a nice addition 👍